Dark matter 'wind' may be warped by the sun

Dark-matter hunters may need to check their calendars. The sun's gravity could change the time when dark matter signals are detected on Earth, which could help sharpen the search for the elusive substance.

Invisible dark matter is thought to make up most of the matter in the universe. Physicists hope to detect it in the form of weakly interacting massive particles (WIMPs) when they collide with ordinary matter in underground detectors.

Some have argued that the rate of such interactions should vary with the seasons, as Earth's orbit brings it ploughing through the cloud of dark matter suffusing the galaxy. When the planet heads into this "WIMP wind", around 1 June, we should see more dark matter strikes; in December, when Earth is moving downwind, we should see fewer.

Warp factor

Now, Benjamin Safdi of Princeton University and his colleagues note something that all experiments have neglected: the sun. As WIMPs stream through the solar system, the sun's gravity bends their trajectories, focusing the streaming particles on a particular location in Earth's orbit. This effect can shift the date of the maximum number of collisions by anything from a few days up to several months, depending on the WIMPs' mass and speed. "This force warps the dark matter 'wind' in a way that had not previously been noticed," Safdi says.

The fact that the date of maximum WIMP collisions should change depending on their energy could lend future searches a sharper scalpel to scrape true dark matter signals away from background noise, he adds.

"Our result gives dark matter direct-detection experiments an excellent way of distinguishing real interactions with the galactic dark matter halo from background," Safdi says. "It is hard to imagine a background source which could mimic this energy-dependent modulation."

Punchline coming

The work does not explain DAMA's possible dark-matter signal, but re-analysing the data using the new approach could help support or refute their results, Safdi says.

"There is already a slight trend in the data consistent with our prediction for the gravitational focusing effect – that is, the date of the maximum moves further away from June 1 at lower energies," says team member Samuel Lee, also of Princeton. "One punchline of our study is that accounting for the gravitational focusing effect can perhaps rule out or confirm the dark-matter interpretation of the DAMA annual modulation."

Richard Gaitskell of Brown University in Providence, Rhode Island, who works on a direct-detection experiment in South Dakota called LUX, says that the new work could be important for helping design future experimental set-ups. "These researchers have clearly demonstrated just how potentially interesting data from a direct-detection experiment can be," he says.

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